As we (ahem) get older, our body sometimes acts as though it’s been possessed by an alien being. We put on weight that we can’t lose. We feel tired all the time. Our skin gets dry. Our hair thins. The list goes on. Plenty of people, especially women, blame these changes on their thyroid. And in an effort to reclaim their waistline and vitality, many are turning to over-the-counter thyroid boosters, a move that has doctors concerned.

A study published in the November 2013 issue of the journal Thyroid found that self-treating with OTC thyroid support supplements may cause thyroid levels to climb to abnormal levels, putting people are at risk for serious problems.

Thyroid 101

The thyroid, a tiny butterfly-shaped gland at the base of the neck, secretes a hormone that controls our metabolism, including our energy level; our weight; our body temperature; our mental health; and our muscle strength. It affects each and every cell, muscle and organ in our body.

And when it slows down, typically as we age, we develop hypothyroidism, or an underactive thyroid. Symptoms include weight gain; constipation; joint pain; fatigue; dry skin; brittle nails; hair loss; and even problems such as high cholesterol, a slow heart rate, carpal tunnel syndrome and depression.

While an underactive thyroid is easily treated with prescription thyroid medication, many people are trying to jumpstart their sluggish gland, or supplement their thyroid medicine, with thyroid “boosters”, without medical supervision.

Researchers recently analyzed 10 commercially available dietary thyroid supplements marketed for “thyroid support.” Half were sold as herbal-based preparations and half contained “raw thyroid” bovine tissue, concentrate or powder. The researchers found that most of the OTC supplements contained measurable amounts of two thyroid hormones—called T4 and T3. (T4, or thyroxine, hormone is prescribed to people with hypothyroidism. T3, or triiodothyronine, may be prescribed as well, though different organs naturally convert T4 into T3.) In fact, some of the supplements contained amounts of T4 or T3 that equaled or exceeded the dosage found in common prescription doses.

Supplement Risks

This is a concern for several reasons. Since the FDA doesn’t evaluate or approve dietary supplements, consumers have no way of knowing if a product contains the ingredients or dosage listed on the label. What’s more, manufacturers of supplements don’t have to submit safety information to the FDA, so there’s no way to track problems caused by the products. And since people may take supplements along with OTC or prescription medicines, they’re at risk for potentially dangerous drug-supplement interactions.

According to Dr. Lee, thyroid medication has a narrow therapeutic index. Because of this, people need to have their thyroid levels measured before their doctor writes a prescription. Then the dosage of the medication has to be precisely calibrated to raise thyroid levels to a healthy level. What’s more, each patient’s levels need to be monitored to make sure they stay within a healthy range.

If you don’t have a thyroid problem and take a thyroid supplement, your thyroid levels will climb. Ditto if you are taking prescription thyroid medication and a supplement. The result: Hyperthyroidism, or an overactive thyroid. People with this condition may experience symptoms such as heart palpitations, trouble sleeping, anxiety, excess sweating, weight loss, insatiable hunger, an intolerance for heat, hand tremors and diarrhea. That’s not all.

Excess thyroid hormone can cause bones to lose calcium, according to Dr. Lee. “That’s a known risk factor for osteoporosis,” she says. Another risk: “Hyperthyroidism makes your heart beat harder and faster,” she explains. That could affect blood flow to the heart, upping the risk for a heart attack.

The good news: Once you stop taking thyroid support supplements, your thyroid levels will drop and these symptoms should ease.

So-called thyroid support supplements have experts concerned for another reason. The study researchers found that some contained iodine-rich seaweed such as kelp or bladderwrack.

While the thyroid needs iodine to produce thyroid hormone, we get plenty from our diet. “If you get four cups of milk a day, you get all the iodine you need,” says Dr. Lee. That’s also the case if you use iodized salt sparingly when you cook, she adds. The extra iodine found in supplements may be enough to put you in the too-much-iodine range. “A little bit of iodine is good for you but a lot will shut down the thyroid if you have mild hypothyroidism,” says Dr. Lee.

These supplements may also affect your thyroid treatment. “I have some patients whose thyroid function is never in the normal range and I never know why,” says Dr. Lee. She suspects they may be taking thyroid support supplements and not telling her. “A patient’s thyroid levels will be too high and I will adjust her medication,” she says. “Or a patient will stop taking the supplements and her thyroid levels will go down.”

To lose weight or boost energy, Dr. Lee recommends lifestyle changes, not supplements. “I do a lot of weight and fatigue management,” she says. “If your thyroid function is normal, you are not tired and not losing weight because of your thyroid.”

“It is part of natural aging to gain weight if you don’t reduce calories and increase your exercise,” she explains. And, she adds, if you don’t make sleep a priority, then you are going to be tired.